One day in 2007, Dr. Giulio Tononi lay on a hospital stretcher as an anesthesiologist prepared him for surgery. For Dr. Tononi, it was a moment of intellectual exhilaration. He is a distinguished chair in consciousness science at the University of Wisconsin, and for much of his life he has been developing a theory of consciousness. Lying in the hospital, Dr. Tononi finally had a chance to become his own experiment.

The anesthesiologist was preparing to give Dr. Tononi one drug to render him unconscious, and another one to block muscle movements. Dr. Tononi suggested the anesthesiologist first tie a band around his arm to keep out the muscle-blocking drug. The anesthesiologist could then ask Dr. Tononi to lift his finger from time to time, so they could mark the moment he lost awareness.

The anesthesiologist did not share Dr. Tononi’s excitement. “He could not have been less interested,” Dr. Tononi recalled. “He just said, ‘Yes, yes, yes,’ and put me to sleep. He was thinking, ‘This guy must be out of his mind.’ ”

Dr. Tononi was not offended. Consciousness has long been the province of philosophers, and most doctors steer clear of their abstract speculations. After all, debating the finer points of what it is like to be a brain floating in a vat does not tell you how much anesthetic to give a patient.

But Dr. Tononi’s theory is, potentially, very different. He and his colleagues are translating the poetry of our conscious experiences into the precise language of mathematics. To do so, they are adapting information theory, a branch of science originally applied to computers and telecommunications. If Dr. Tononi is right, he and his colleagues may be able to build a “consciousness meter” that doctors can use to measure consciousness as easily as they measure blood pressure and body temperature. Perhaps then his anesthesiologist will become interested.

“I love his ideas,” said Christof Koch, an expert on consciousness at Caltech. “It’s the only really promising fundamental theory of consciousness.”

Dr. Tononi’s obsession with consciousness started in his teens. He was initially interested in ethics, but he decided that questions of personal responsibility depended on our consciousness of our own actions. So he would have to figure out consciousness first. “I’ve been stuck with this thing for most of my life,” he said.

Eventually he decided to study consciousness by becoming a psychiatrist. An early encounter with a patient in a vegetative state convinced Dr. Tononi that understanding consciousness was not just a matter of philosophy.

“There are very practical things involved,” Dr. Tononi said. “Are these patients feeling pain or not? You look at science, and basically science is telling you nothing.”

Dr. Tononi began developing models of the brain and became an expert on one form of altered consciousness we all experience: sleep. In 2000, he and his colleagues found that Drosophila flies go through cycles of sleeping and waking. By studying mutant flies, Dr. Tononi and other researchers have discovered genes that may be important in sleep disorders.

For Dr. Tononi, sleep is a daily reminder of how mysterious consciousness is. Each night we lose it, and each morning it comes back. In recent decades, neuroscientists have built models that describe how consciousness emerges from the brain. Some researchers have proposed that consciousness is caused by the synchronization of neurons across the brain. That harmony allows the brain to bring together different perceptions into a single conscious experience.

Dr. Tononi sees serious problems in these models. When people lose consciousness from epileptic seizures, for instance, their brain waves become more synchronized. If synchronization were the key to consciousness, you would expect the seizures to make people hyperconscious instead of unconscious, he said.

While in medical school, Dr. Tononi began to think of consciousness in a different way, as a particularly rich form of information. He took his inspiration from the American engineer Claude Shannon, who built a scientific theory of information in the mid-1900s. Mr. Shannon measured information in a signal by how much uncertainty it reduced. There is very little information in a photodiode that switches on when it detects light, because it reduces only a little uncertainty. It can distinguish between light and dark, but it cannot distinguish between different kinds of light. It cannot tell the differences between a television screen showing a Charlie Chaplin movie or an ad for potato chips. The question that the photodiode can answer, in other words, is about as simple as a question can get.

Our neurons are basically fancy photodiodes, producing electric bursts in response to incoming signals. But the conscious experiences they produce contain far more information than in a single diode. In other words, they reduce much more uncertainty. While a photodiode can be in one of two states, our brains can be in one of trillions of states. Not only can we tell the difference between a Chaplin movie and a potato chip, but our brains can go into a different state from one frame of the movie to the next.

“One out of two isn’t a lot of information, but if it’s one out of trillions, then there’s a lot,” Dr. Tononi said.

Consciousness is not simply about quantity of information, he says. Simply combining a lot of photodiodes is not enough to create human consciousness. In our brains, neurons talk to one another, merging information into a unified whole. A grid made up of a million photodiodes in a camera can take a picture, but the information in each diode is independent from all the others. You could cut the grid into two pieces and they would still take the same picture.

Consciousness, Dr. Tononi says, is nothing more than integrated information. Information theorists measure the amount of information in a computer file or a cellphone call in bits, and Dr. Tononi argues that we could, in theory, measure consciousness in bits as well. When we are wide awake, our consciousness contains more bits than when we are asleep.

Photo

BRAIN METRICS Dr. Giulio Tononi wants to build a “consciousness meter.”Credit
Narayan Mahon for The New York Times

For the past decade, Dr. Tononi and his colleagues have been expanding traditional information theory in order to analyze integrated information. It is possible, they have shown, to calculate how much integrated information there is in a network. Dr. Tononi has dubbed this quantity phi, and he has studied it in simple networks made up of just a few interconnected parts. How the parts of a network are wired together has a big effect on phi. If a network is made up of isolated parts, phi is low, because the parts cannot share information.

But simply linking all the parts in every possible way does not raise phi much. “It’s either all on, or all off,” Dr. Tononi said. In effect, the network becomes one giant photodiode.

Networks gain the highest phi possible if their parts are organized into separate clusters, which are then joined. “What you need are specialists who talk to each other, so they can behave as a whole,” Dr. Tononi said. He does not think it is a coincidence that the brain’s organization obeys this phi-raising principle.

An error has occurred. Please try again later.

You are already subscribed to this email.

Dr. Tononi argues that his Integrated Information Theory sidesteps a lot of the problems that previous models of consciousness have faced. It neatly explains, for example, why epileptic seizures cause unconsciousness. A seizure forces many neurons to turn on and off together. Their synchrony reduces the number of possible states the brain can be in, lowering its phi.

Dr. Koch considers Dr. Tononi’s theory to be still in its infancy. It is impossible, for example, to calculate phi for the human brain because its billions of neurons and trillions of connections can be arranged in so many ways. Dr. Koch and Dr. Tononi recently started a collaboration to determine phi for a much more modest nervous system, that of a worm known as Caenorhabditis elegans. Despite the fact that it has only 302 neurons in its entire body, Dr. Koch and Dr. Tononi will be able make only a rough approximation of phi, rather than a precise calculation.

“The lifetime of the universe isn’t long enough for that,” Dr. Koch said. “There are immense practical problems with the theory, but that was also true for the theory of general relativity early on.”

Dr. Tononi is also testing his theory in other ways. In a study published this year, he and his colleagues placed a small magnetic coil on the heads of volunteers. The coil delivered a pulse of magnetism lasting a tenth of a second. The burst causes neurons in a small patch of the brain to fire, and they in turn send signals to other neurons, making them fire as well.

To track these reverberations, Dr. Tononi and his colleagues recorded brain activity with a mesh of scalp electrodes. They found that the brain reverberated like a ringing bell, with neurons firing in a complex pattern across large areas of the brain for 295 milliseconds.

Then the scientists gave the subjects a sedative called midazolam and delivered another pulse. In the anesthetized brain, the reverberations produced a much simpler response in a much smaller region, lasting just 110 milliseconds. As the midazolam started to wear off, the pulses began to produce richer, longer echoes.

These are the kinds of results Dr. Tononi expected. According to his theory, a fragmented brain loses some of its integrated information and thus some of its consciousness. Dr. Tononi has gotten similar results when he has delivered pulses to sleeping people — or at least people in dream-free stages of sleep.

In this month’s issue of the journal Cognitive Neuroscience, he and his colleagues reported that dreaming brains respond more like wakeful ones. Dr. Tononi is now collaborating with Dr. Steven Laureys of the University of Liège in Belgium to test his theory on people in persistent vegetative states. Although he and his colleagues have tested only a small group of subjects, the results are so far falling in line with previous experiments.

If Dr. Tononi and his colleagues can get reliable results from such experiments, it will mean more than just support for his theory. It could also lead to a new way to measure consciousness. “That would give us a consciousness index,” Dr. Laureys said.

Traditionally, doctors have measured consciousness simply by getting responses from patients. In many cases, it comes down to questions like, “Can you hear me?” This approach fails with people who are conscious but unable to respond. In recent years scientists have been developing ways of detecting consciousness directly from the activity of the brain.

In one series of experiments, researchers put people in vegetative or minimally conscious states into fMRI scanners and asked them to think about playing tennis. In some patients, regions of the brain became active in a pattern that was a lot like that in healthy subjects.

Dr. Tononi thinks these experiments identify consciousness in some patients, but they have serious limitations. “It’s complicated to put someone in a scanner,” he said. He also notes that thinking about tennis for 30 seconds can demand a lot from people with brain injuries. “If you get a response I think it’s proof that’s someone’s there, but if you don’t get it, it’s not proof of anything,” Dr. Tononi said.

Measuring the integrated information in people’s brains could potentially be both easier and more reliable. An anesthesiologist, for example, could apply magnetic pulses to a patient’s brain every few seconds and instantly see whether it responded with the rich complexity of consciousness or the meager patterns of unconsciousness.

Other researchers view Dr. Tononi’s theory with a respectful skepticism.

“It’s the sort of proposal that I think people should be generating at this point: a simple and powerful hypothesis about the relationship between brain processing and conscious experience,” said David Chalmers, a philosopher at Australian National University. “As with most simple and powerful hypotheses, reality will probably turn out to be more complicated, but we’ll learn something from the attempt. I’d say that it doesn’t solve the problem of consciousness, but it’s a useful starting point.”

Dr. Tononi acknowledged, “The theory has to be developed a bit more before I worry about what’s the best consciousness meter you could develop.” But once he has one, he would not limit himself to humans. As long as people have puzzled over consciousness, they have wondered whether animals are conscious as well. Dr. Tononi suspects that it is not a simple yes-or-no answer. Rather, animals will prove to have different levels of consciousness, depending on their integrated information. Even C. elegans might have a little consciousness.

“Unless one has a theory of what consciousness is, one will never be able to address these difficult cases and say anything meaningful,” Dr. Tononi said.

A version of this article appears in print on September 21, 2010, on Page D1 of the New York edition with the headline: Sizing Up Consciousness By Its Bits. Order Reprints|Today's Paper|Subscribe